The invention relates generally to embossing foils and more particularly to hot embossing foils.
U.S. Pat. No. 4,376,006 discloses an embossing foil construction comprising a backing foil and a transfer layer which can be removed therefrom and which comprises at least a magnetic layer consisting of a dispersion of magnetisable particles in a binding agent. A metal layer and a layer of lacquer are provided on the side of the magnetic layer which is towards the backing foil. That design seeks to afford the possibility of the magnetic embossing foil also being produced in bright attractive colours, for which purpose the magnetic layer, which is generally dark, is covered by the metal layer. Purely decorative purposes are therefore the consideration in that respect.
It is conventional practice for for example cheque cards, credit cards, savings books and similar security documents and items to be provided with magnetic strips, strips which can be signed thereon, or structures which have a particular optical effect in order thereby to make it possible to store certain items of data or to enhance the safeguards against forgeries. An advantage when using magnetic strips is that it is very easily possible to provide for suitable storage of relevant data in the magnetic strip. However, that design suffers from the disadvantage that forgery or erasure is a comparatively simple matter, with the usual forms of magnetic strips. In principle that depends on the way in which the magnetic strip is applied. It is true to say that for many areas of use, it has been found to be advantageous for the magnetic strips to be applied by means of embossing foils, in particular hot-process embossing foils, because in that case the strip can be easily applied to the security document or like item, and it is even possible for the magnetic strip to be coded before it is applied to the document or card in question.
Various ways have already been investigated, for increasing the safeguards against forgeries in respect of documents or security cards or the like bearing magnetic strips. In that connection, one known process provides for forming in the coating of the magnetic strip, a pattern of magnetisable pigments, which is directed at an angle of 45.degree. with respect to the longitudinal direction. That 45.degree. basic pattern in regard to the magnetisation effect cannot be altered at a level of cost such as to make it usually worthwhile for forgery purposes.
Another possible process lies in using a double-layer magnetic strip, wherein one layer has a high level of coercive force, that is to say, it carries items of information which are difficult to change, while the other layer which is usually on top of the first-mentioned layer receives the erasable data with a low level of coercive force. However, double-layer magnetic strips of that kind are comparatively difficult to produce and also give rise to problems in regard to encoding thereof.
Attempts have also already been made to provide the magnetic layer of magnetic strips with special graphic characters or symbols which consist of pigmented lacquers, in which respect both pigments which can be detected in visible light and also pigments which can be detected only in invisible light have been used. However, such a procedure suffers from the disadvantage that the pigmented layers must be comparatively thick so that, in order to produce a sufficient signal voltage, the magnetic layer must also be comparatively thick. However, difficulties are encountered in seeking to work tidily and cleanly with embossing foils which have a very thick magnetic layer and also a thick pigmented layer. It should also be borne in mind that, in order to be able to code a foil, the surface thereof must be very flat and even. That is also an aspect which leaves something to be desired when using pigmented lacquers for producing the pattern designed to prevent forgeries, as the surface produced is comparatively wavy. Such a wavy surface configuration also suffers from the disadvantage that there must necessarily be a comparatively large distance between the magnetic layer and the reading head, whicn in turn also gives rise to a reduction in the signal voltage. There is also the danger that a rough surface may result in premature damage to the reading head of the reading device used.
Finally, attempts have also already been made to use structures which have an optical diffraction effect and in particular which have a holographic effect, for the purposes of characterising and identifying security documents, cards and the like items. In the past, those structures have predominantly been embossed directly into cards made from plastic material. The attempt has also already been made to provide embossing foils with diffraction structures having a holographic effect (phase holograms), but hitherto no further identification or characterisation means or information has been included in the foils, besides the diffraction structures. The use of such structures gives a high level of safeguard against forgery because that structure is severely damaged or destroyed in the event of an attempt being made to strip off the corresponding foil. Furthermore, the diffraction structure can be easily read by machine, by means of lasers or LEDs. However, there are also holograms which can be detected with the naked eye. Furthermore, by means of a particular arrangement, in particular in regard to reflection holograms, it is possible to provide that characteristic changes in the diffraction images can be produced even with different conditions of illumination, in daylight and artificial light, whereby it is possible to provide a visually clearly detectable feature indicating authenticity in respect of a document. However, a disadvantage of such an arrangement is that it is not possible to alter the information once it has been stored on the document. It is at best possible to produce erasure of the diffraction structure over a certain area, but that is then irreversible.